This invention relates to the production of high purity fused silica glass, and in particular, to methods and apparatus for minimizing the build-up of glass deposits around the burner hole in a high purity fused silica refractory furnace.
In overview, high purity fused silica glass is made by depositing fine particles of silica on a sand bait in a refractory furnace at temperatures exceeding 1650xc2x0 C. The silica particles are generated in a flame when a silicon containing raw material along with natural gas is passed through a burner into the furnace chamber. These particles are deposited on the hot surface of a rotating body containing a sand bait, where they consolidate into a very viscous fluid which is later cooled to the glassy (solid) state. The rotating body is in the form of a refractory cup or containment vessel which is used to provide insulation to the glass as it builds up, and the furnace cavity formed by the cup interior and the crown of the furnace is kept at high temperatures. In the art, glass making procedures of this type are known as vapor phase hydrolysis-oxidation processes, or simply as flame hydrolysis processes. The body formed by the deposited particles is often referred to as a xe2x80x9cboulexe2x80x9d and it is understood that this terminology includes any silica-containing body formed by a flame hydrolysis process.
FIG. 1 shows a prior art furnace 10 for producing fused silica glass. The furnace 10 includes an outer ring wall 12 which supports a crown 14. The crown 14 is provided with a plurality of burner holes 16, of which one is shown in the drawing. Each burner hole 16 has a burner 18 positioned thereabove at an inlet end for directing a flame through the burner hole 16 and into the cavity 20 of the furnace 10. The furnace 10 is provided with a rotatable base 22, which with containment wall 24 forms a cup or containment vessel 26. The rotatable base 22, forming the bottom of the cup-like containment vessel 26, is covered with high purity bait sand 28 which collects the initial silica particles forming the boule 30. A shadow wall 32 is provided between the containment wall 24 and the outer ring wall 12, and a suitable seal 34 is formed between the shadow wall 32 and the outer ring wall 12 to prevent the infiltration of air between such wall portions into the furnace cavity 20.
However, as indicated in FIG. 1, the design of the furnace is such that room air may be easily infiltrated into the furnace through the burner holes 16 in the crown 14 of the furnace and through the gap 36 formed between containment wall 24 and shadow wall 32, as shown by arrows a and b, respectively. The infiltrated air reduces the concentration of hydrogen in the furnace atmosphere in two ways. Firstly, it physically dilutes the combustion gases and secondly, it brings in additional oxygen that reacts with the hydrogen to form water vapor. The overall result of the air entrained is a reduction in dissolved hydrogen in the glass. At the same time, the air infiltrated through the burner hole provides the oxygen necessary to react with unburned hydrogen and CO furnace gases within the furnace cavity that are re-circulated close to the burner hole.
The quality of the high purity fused silica boule can be improved by increasing the amount of hydrogen dissolved in the boule. One method of increasing hydrogen in the glass can be accomplished by increasing the hydrogen in the furnace atmosphere. Hydrogen is produced as an intermediate species during the combustion of the organic raw material of silica and the methane gas. It is known from flame chemistry calculations that burning a fuel-rich mixture of fuel and oxygen can increase hydrogen in the furnace atmosphere. However, on burning such a fuel-rich mixture, it was found that the overall run time of the furnace is significantly decreased because of a build-up of glass around the burner hole. The shorter run times of the furnace due to the glass build-up has created problems which adversely affect the glass properties, such as transmission.
FIG. 2 illustrates the problem which we have identified that causes undesirable build-up of silica particles around the burner hole. The flow fields of the various gases are shown by arrows a, c, and d in FIG. 2. A back-flow of furnace gases, such as unburned CO and H2, into the burner hole 16, and about the outer periphery of the flame F is shown by arrows c. The back-flow shown by arrows c passes close to a hot bottom edge or exit end 15 of the burner hole 16. When the fuel/oxygen mixture is fuel-rich, such conditions result in partial combustion of the fuel and leaves substantial amounts of unburned CO and H2 in the furnace atmosphere that are circulated such as shown by arrows d within the burner hole 16.
Therefore, when the unburned CO and H2 are pulled into the burner hole, such gases encounter oxygen from the crown flow shown by arrows a and thus such gases burn very close to the burner hole exit end or rim 15. Since the back-flow stream c already contains silica particles, a locally high temperature region around the burner hole exit end causes silica particles to deposit and form a glassy build-up 38 at an exit end 15 of the burner hole 16 at a junction with a bottom edge of the crown 14 within the cavity 20 of the furnace 10. It thus became an object of the invention to minimize the combustion of re-circulated hydrogen and carbon monoxide furnace gases close to the burner hole exit end, and several approaches were entertained including the use of a gaseous curtain to minimize oxygen concentrations next to the burner hole sidewalls. The use of gaseous curtains are known for unrelated purposes, such as for preventing the contamination of wafers in semiconductor manufacturing as shown in U.S. Pat. Nos. 4,803,948 and 4,950,156; and for cooling burner nozzles with oxygen as shown in U.S. Pat. Nos. 4,303,386 and 5,567,141.
It is apparent that there is a need for not only recognizing a problem of glass build-up about the burner hole in high purity fused silica processes, but also for a solution to such problem while maintaining high quality fused silica glass boules.
In view of the foregoing, it is an object of this invention to provide improved method and apparatus for producing silica-containing boules by flame hydrolysis. In particular, it is an object of the invention to inhibit or minimize glass build-up around the burner hole in high purity fused silica processes. It is a further object of the invention to minimize the combustion of recirculated furnace gases including hydrogen and carbon monoxide, close to the burner hole exit rim in a furnace for producing high purity fused silica, by reducing the concentration of oxygen in the vicinity of the burner hole rim through the use of an inert gas curtain between the burner flame and the sidewalls of the burner hole.
FIG. 1 is a fragmental schematic diagram in elevation of a prior art furnace used to produce fused silica boules using a flame hydrolysis process.
FIG. 2 is an enlarged fragmental view in elevation of a portion of the furnace shown in FIG. 1.
FIG. 3 is an enlarged fragmental view similar to FIG. 2, but showing the improvement of the present invention.
FIG. 4 is a slightly enlarged plan view of the burner and inert gas ring taken along line 4xe2x80x944 of FIG. 3.